1. Field of the Invention
The present disclosure relates to image-forming apparatuses such as a laser beam printer (LBP), a digital copier, and a digital facsimile. Specifically, the present disclosure relates to an image-forming apparatus that performs optical writing by using a laser beam.
2. Description of the Related Art
An electrophotographic image-forming apparatus includes an optical scanning unit for exposing a photosensitive member. The optical scanning unit emits laser light on the basis of image data, causes the laser light to be reflected by a rotational polygon mirror, causes the reflected light to be transmitted through a scanning lens, and thereby irradiates the photosensitive member with the transmitted light so that the photosensitive member is exposed. With the rotational polygon mirror being rotated, scanning is performed by moving a spot of laser light formed on the surface of the photosensitive member, and thereby a latent image is formed on the photosensitive member.
A scanning lens is a lens having a so-called fθ characteristic. The fθ characteristic is an optical characteristic in which laser light is focused on the surface of a photosensitive member so that the spot of the laser light moves on the surface of the photosensitive member at a uniform velocity when a rotational polygon mirror is rotating at a uniform angular velocity. With use of a scanning lens having such an fθ characteristic, appropriate exposure can be performed.
The size and cost of a scanning lens having such an fθ characteristic are relatively large. Thus, for the purpose of reducing the size and cost of an image-forming apparatus, it is suggested not to use a scanning lens or to use a scanning lens that does not have an fθ characteristic.
Japanese Patent Laid-Open No. 58-125064 discloses a technique of performing electrical correction to change an image clock frequency during one scanning operation, so that dots formed on the surface of a photosensitive member have a constant width even if the spot of laser light does not move at a uniform velocity on the surface of the photosensitive member.
However, even if a scanning lens having an fθ characteristic is not used and the above-described electrical correction is performed to make the widths of individual dots constant, the time for forming one dot varies at an end portion and a center portion in the main scanning direction, for example. That is, the velocity at which the spot of laser light moves on the surface of the photosensitive member to form a dot varies in the case of forming a dot at an end portion in the main scanning direction and in the case of forming a dot at a center portion in the main scanning direction. Therefore, the amount of exposure per unit area on a drum surface differs between a dot at the end portion in the main scanning direction and a dot at the center portion in the main scanning direction, and the difference in the amount of exposure may cause image failure.